A conventional electronic sensor used to detect a phenomenon such as pressure, temperature, force and acceleration typically incorporates a primary transducer having an electrical characteristic which varies in response to the phenomenon, such as a variable resistor strain gauge element, a piezoelectric element or the like. The primary transducer is mounted at the location where the phenomenon is to be detected, and is connected by electrical conductors to a secondary signal processing or recording device for converting an electrical signal from the primary transducer into useful information. Electronic sensors are susceptible to problems such as electromagnetic interference from other components, cross-talk between conductors, corrosion, poor sensitivity and poor reliability.
To alleviate these problems, fiber optic sensing systems have been proposed. These systems generally incorporate a sensor, a light source, an optical monitoring device and one or more optical fibers for connecting the sensor to the source and the monitoring device. The sensor typically includes an element which can interact with light from the source in a manner which varies dependent upon the phenomenon to be detected. Thus, light from the source is transmitted through the optical fiber to the sensor, and is then modified at the sensor in accordance with the phenomenon to be detected. The modified light is transmitted back through the optical fiber to the monitoring device, which in turn detects the change in the light and provides information relating to the phenomenon. Merely by way of example, the sensor may include a reflective element arranged so that the position of the reflective element, and hence the phase of the reflected light, changes in response to the phenomenon to be detected. The monitoring device may be arranged to monitor the phase of the reflected light.
Fiber optic systems offer significant advantages over alternative systems such as conventional electromagnetic systems. An optical fiber can be described as a glass or ceramic conduit through which light waves are permitted to travel. The optical fiber should be designed so that only a small percentage of the light waves transmitted therein will be permitted to exit through the side walls of the fiber.
A typical optical fiber includes a core, having a predetermined refractive index, and cladding entirely surrounding the core, having a predetermined refractive index greater than that of the core. Such an arrangement permits light waves to be guided through the core of the optical fiber with little leakage through the fiber wall. The refractive index represents the ratio of the velocity of light, which passes through the optical fiber material, to the velocity of light passing through a vacuum which is known to be represented by a constant, i.e., 3.0.times.10.sup.8 m/s. Thus, the velocity that light waves travel through a particular material is inversely related to the index of refraction. In this regard, the velocity at which light waves travel through a particular material is lower for materials having a relatively high index of refraction than it is for materials having a relatively low index of refraction.
The advantages of fiber optic sensors over conventional sensors have spurred inventors to expend a great deal of time and effort in attempting to develop new fiber optic sensors that are effective for use in various applications, and methods of manufacturing the same in large quantities and at a low cost. Indeed, the prior art is replete with such attempts. However, an analysis of such prior art indicates that additional problems have arisen. One problem which has particularly plagued prior art fiber optic sensors is the need for precise alignment of plural substrates constituting the sensor with one another and/or with the optical fiber.
In this regard, U.S. Pat. No. 5,087,124 to Smith et al. discloses a fiber optic interferometric pressure sensor and a method of manufacturing same. The sensor is manufactured by initially applying anisotropic etching techniques to separate silicon substrates. An optical fiber is then sandwiched between the two substrates. One of the substrates includes a deflectable membrane, and the other substrate includes an immobile reflective surface obliquely arranged with respect to the end face of the optical fiber. The specification of the Smith, et al. patent specifically states that it is particularly important for the immobile reflective surface on one of the substrates to be precisely aligned with the deflectable membrane on a second substrate and the end of the optical fiber (see Col. 4, line 64 - col. 5, line 7).
U.S. Pat. No. 4,916,497 to Gaul et al. discloses an integrated circuit including an optical fiber and a method of fabrication in which the circuits are formed by bonding two substrates together and inserting an optical fiber therebetween. A reflective surface is formed on one of the substrates and is arranged at an obtuse angle with respect to the end face of the optical fiber.
U.S. Pat. No. 5,052,228 to Haritonidis discloses a shear stress measuring device including a deflectable diaphragm arranged in a plane perpendicular to the axis of an optical fiber. With regard to fiber optic Fabry-Perot temperature sensors, results of experiments showing favorable sensor performance were reported in Lee, et al., "Fiber Optic Fabry-Perot Temperature Sensor Using a Low-Coherence Light Source," Journal of Light Wave Technology, vol. 9, no. 1, pp. 129-133, January 1991.
Despite these and other efforts in the art, there has been a considerable need for an improved fiber optic interferometric sensor, and methods of manufacturing same. The present invention addresses these needs.